(1) Field of the Invention
The present invention relates to coin telephone line circuits and more particularly to a circuit for monitoring the current thresholds associated with coin telephone operations.
(2) Description of the Prior Art
Previous methods of monitoring coin telephone operations include relay and electronic circuits. Relay circuits have the drawbacks of high cost, large space requirements, contact wear problems, and still require interface circuitry to make their output compatible with digital sense circuitry. Electronic sensors require complex electronic circuits of a large number of components and must be connected to the telephone line thereby being subject to high voltage transients from lightening and high inductance voltage spikes from the coin relay. Even the most recent electronic sensors, which use optical couplers to isolate the line from the digital sense logic, are susceptible to high voltage failure since they require a solid state diode LED to be connected to the line.
Accordingly, it is the object of the present invention to provide a coin telephone monitor circuit using a minimal amount of highly reliable electronic circuitry which is completely isolated from the telephone line and thereby not susceptible to high voltage transients.